One goal of a network manager is to control total cost of ownership of the network. Cabling problems can cause a significant amount of network downtime and can require troubleshooting resources, which increase the total cost of ownership. Providing tools that help solve cabling problems more quickly will increase network uptime and reduce the total cost ownership.
Referring now to FIG. 1, conventional cable testers 10 are frequently used to isolate cabling problems. The cable testers 10 are coupled by a connector 12 (such as an RJ-45 or other connector) to a cable 14. A connector 15 connects the cable to a load 16. Conventional cable testers typically require the load 16 to be a remote node terminator or a loop back module. Conventional cable tests may generate inaccurate results when the cable is terminated by an active link partner that is generating link pulses during a test. The cable tester 10 performs cable analysis and is able to detect a short, an open, a crossed pair, or a reversed pair. The cable tester 10 can also determine a cable length to a short or open.
A short condition occurs when two or more lines are short-circuited together. An open condition occurs when there is a lack of continuity between ends at both ends of a cable. A crossed pair occurs when a pair is connected to different pins at each end. For example, a first pair is connected to pins 1 and 2 at one end and pins 3 and 6 at the other end. A reversed pair occurs when two ends in a pair are connected to opposite pins at each end of the cable. For example, a line on pin 1 is connected to pin 2 at the other end. A line on pin 2 is connected to pin 1 at the other end.
The cable tester 10 employs time domain reflection (TDR), which is based on transmission line theory, to troubleshoot cable faults. The cable tester 10 transmits a pulse 17 on the cable 14 and measures an elapsed time until a reflection 18 is received. Using the elapsed time and a cable propagation constant, a cable distance can be estimated and a fault can be identified. Two waves propagate through the cable 14. A forward wave propagates from a transmitter in the cable tester 10 towards the load 16 or fault. A return wave propagates from the load 16 or fault to the cable tester 10.
A perfectly terminated line has no attenuation and an impedance that is matched to a source impedance. The load is equal to the line impedance. The return wave is zero for a perfectly terminated line because the load receives all of the forward wave energy. For open circuits, the return wave has an amplitude that is approximately equal to the forward wave. For short circuits, the return wave has a negative amplitude is also approximately equal to the forward wave.
In transmission line theory, a reflection coefficient is defined as:
      T    L    =            R_wave      F_wave        =                            V          -                          V          +                    =                                    Z            L                    -                      Z            O                                                Z            L                    +                      Z            O                              Where ZL is the load impedance and ZO is the cable impedance. The return loss in (dB) is defined as:
            R      L        ⁢                  ⁢          (      db      )        =            20      ⁢                          ⁢              LOG        10            ⁢                          ⁢                                1                      T            L                                        =          20      ⁢              LOG        10            ⁢                          ⁢                                                          Z              L                        +                          Z              O                                                          Z              L                        -                          Z              O                                                  Return loss performance is determined by the transmitter return loss, the cable characteristic impedance and return loss, and the receiver return loss. IEEE section 802.3, which is hereby incorporated by reference, specifies receiver and transmitter minimum return loss for various frequencies. Additional factors that may affect the accuracy of the return loss measurement include connectors and patch panels. Cable impedance can also vary, for example CAT5 UTP cable impedance can vary ±15 Ohms.